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Original articles

Fetal head–perineum distance measured by transperineal ultrasound imaging as a predictive factor for successful induction of labor

Abdel Hafiz, Ali

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Evidence Based Women's Health Journal: May 2014 - Volume 4 - Issue 2 - p 105-107
doi: 10.1097/01.EBX.0000440897.30442.22
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Abstract

Introduction

Induction of labor is a common obstetric intervention, performed in about 20% of pregnancies 1. Preinduction cervical length, measured by transvaginal sonography, has been shown to have a significant association with the induction-to-delivery interval and with the risk for cesarean section (CS) 2. In the management of labor, there is extensive evidence that digital pelvic examination does not provide accurate assessment of the position and descend of the fetal head, both during the first and also the second stage of labor. Several recent studies using both two-dimensional and three-dimensional ultrasound have now described the objective measures of progression of the fetal head position and descend during labor 3.

Ultrasound measurements of the fetal head–perineum distance have not been evaluated as a predictive factor in women scheduled for induction of labor, but it was found to predict labor outcome in women with prelabor rupture of membranes at term 4. The aim of this study was to evaluate fetal head–perineum distance measured by transperineal ultrasound imaging as a predictive factor for successful induction of labor in pregnant women with engaged head.

Patients and methods

The present study was a prospective clinical trial performed from April 2010 to July 2011. Pregnant singleton women were included if the gestational ages were more than 37 weeks according to last menstrual period or mid-trimester scan if the patient is not sure of date. Induction of labor was indicated as postdate, mild pregnancy-induced hypertension, prelabor rupture of membranes, intrauterine growth restriction, or diabetes mellitus without macrosomia. We excluded women in labor with nonengaged head, previous CS, any scar in uterus, presence of obstetric causes interfering with induction of labor, placenta previa, major fetal anomaly, and grand multipara.

Methods

All women were subjected to complete history taking and general and local vaginal examination. Cervical assessment was carried out including cervical dilatation (cm), cervical effacement (%), station of presenting part, and cervical consistency and position (using modified Bishop score system).

Ultrasound assessment

All participants in this study were subjected to transabdominal ultrasound for determination of fetal gestational age, presentation, and occiput position. In addition, all participants were subjected to transvaginal ultrasound for assessment of cervical length.

Then, all participants of the study were subjected to transperineal ultrasound to measure fetal head–perineal distance while the patient was in the supine position and with empty bladder. The fetal head–perineal distance was measured as the shortest distance from the outer bony limit of fetal skull to the skin surface of the perineum by the transperineal ultrasound examination in transverse view with the probe held over the ischial tuberosity with firm pressure but without creating any discomfort for the women, and the transducer was moved and angled until the shortest distance to the fetal head was visualized as described by Eggebø et al.5.

The midpoint of the pelvic canal is at the level of the ischial spine. The distance from the perineum to the ischial spine is 5 cm according to the WHO stages of head descent. A cutoff level of 45 mm was therefore used for head engagement, corresponding to a fetal head position beneath the ischial spine, the fetal head having passed the narrowest part of the pelvic canal.

Induction of labor

Induction of labor was performed as follows: if the cervix was favorable for induction (Bishop score>6), labor was induced with amniotomy followed by oxytocin drips. If the cervix was unfavorable, cervical ripening was performed with 25 μg vagiprost applied vaginally every 4 h for maximum of four doses until the cervix became favorable, Bishop score being greater than 6, then continue as mentioned above by rupture of membrane and oxytocin drops, when contractions became regular (2–3 contractions/10 min and the cervix became 3–4 cm) to determine the end of the latent phase and start of the active phase. Thereafter, determining the mode of deliveries ‘either vaginally or CS’ and then recording the end results within 24 h of induction of labor. In addition, during observation of the patients, the total doses received, complications, and time of induction–delivery interval was recorded and intrapartum fetal surveillance was performed by intermittent auscultation of fetal heart sound every 15 min. Cardiotocography (CTG) was performed for each case.

Statistical analysis

Data are expressed as mean±SD (range) or as n (%) of cases. Comparison of proportions and means between both groups was made by using the χ2-test and the independent t-test, respectively. The Fisher exact test was used when applicable. The predictive value ultrasound measurements and Bishop scores for a successful vaginal delivery were evaluated using receiver operating characteristic (ROC) curves, and the area under the curve was used as discriminator.

Ethical approval was obtained from the local institutional review board.

Results

In all, 100 pregnant women with an age of 26±4.24 years were included. Success of induction was achieved in 88 women having normal vaginal delivery (NVD), whereas CS was performed in only 12 cases. Areas under the ROC curve for prediction of vaginal delivery were 78% [95% confidence interval (CI), 61–95%] for fetal head–perineum distance (P=0.04), 73% (95% CI, 61–94%) for cervical length (P=0.05), 42% (95% CI, 26–60%) for cervical width (P=0.044), and 59% (95% CI, 34–85%) for Bishop score (P=0.046).

The ROC curves indicated that fetal head–perineum distance of 8.6 cm or less and cervical length of 3.3 cm or less were the best cutoff levels for predicting the mode of delivery. In a multivariate Cox regression analysis, parity, fetal head–perineum distance of 8.6 cm or less, and cervical length of less than 3.3 cm predicted vaginal delivery within 24 h. Pelvic capacity was not a contributing factor.

Discussion

Evaluation of head engagement with perineal ultrasound is a quick, easy to learn, and well-tolerated method to predict induction of labor. When the head is not engaged, the method tends to be less reliable because of fetal movement 5. Other investigators found ultrasound measurements of cervical length and the Bishop score to be of similar value in predicting a cesarean delivery. These results were in accordance with our findings. We chose a cutoff value of 3.3 cm regarding the cervical length. With respect to fetal head–perineum distance, we chose a cutoff value of 8.6 cm – as recommended by many authors – and we found that 26% of the studied pregnant women had fetal head–perineum distance more than 8.6 cm. In contrast, women who had fetal head–perineum distance less than 8.6 cm had more chance for NVD than CS. These data indicated that as the fetal head–perineum distance decrease there is more chance for NVD corresponding to CS delivery and vice versa 6.

Some investigators have pointed out the need for new methods for measuring fetal head descent 7. However, other investigators evaluated ultrasound-measured engagement of the fetal head as a predictive factor for outcome in spontaneous labors by relating the fetal head to the symphysis pubis 8. Fetal head station was determined after digital assessment of the relationship between the fetal head and the maternal ischial spine. Eggebø et al.5 reported that the ischial spine was difficult to visualize with ultrasound imaging. Instead, they used the outlet of the birth canal as a reference line.

Measurements of fetal head–perineal distance may vary with the degree of compression of the soft tissue. The examiner will meet firm resistance when the perineal soft tissue is compressed, without causing discomfort to the woman. The interobserver and intraobserver variability of the method is to be acceptable 5. Thus, they believed that this method could be used to assess fetal head descent, although the relationship with the well-established concept of fetal head station was less clear. An unclear relationship should not undermine the potential clinical value of the proposed method, which is easy and quick to perform.

The method should be validated in new studies for head descent during labor, in particular during protracted labor and before operative vaginal deliveries. The fetal head–perineum distance predicts labor outcome after induction of labor 6. The clinical value of this ultrasound-measured distance was similar to that of ultrasound assessment of cervical length and the Bishop score. Future studies should evaluate if and how these predictive factors can be combined.

Conclusion

Transperineal ultrasound to determine fetal head–perineum distance is an easy and reliable method for prediction of success of induction of labor (Table 1).

T1-10
Table 1:
Relative risk for a vaginal delivery within 24 h after induction according to fetal head–perineum distance and cervical length determined by Cox regression analysis

Acknowledgements

Conflicts of interest

There are no conflicts of interest.

References

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Keywords:

induction of labor; transperineal; transvaginal; ultrasound

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